Process for preparing cadmium red pigment



PROCESS FOR PREPARING CADMIUM RED PIGMENT Benjamin W. Allan and Frank 0. Rummery, Baltimore, Md., assignors to The Glidden Company, Cleveland, Ohio, a corporation of Ohio No Drawing. Application July 19, 1952,

Serial No. 299,920

8 Claims. (Cl. 106-301) thereinto, the color deepens to red, then to maroon, and ultimately to very deep maroons. The extended cadmium red pigments are analogous to the pure reds in the foregoing respects, the difference being that the extended reds include barium sulrate as extender. In the past the selenium which has been introduced into a pure or exmental state, either as very finely ground selenium intimately mixed into the cadmium sulfide, or as elemental selenium coprecipitated with the cadmium sulfide from a solution of an alkali metal sulfide or alkali-earth metal sulfide. We have now found, however, that it is unneces- Another object is to introduce selenium into pure or extended cadmium sulfide greencake by supplying at least extended uncalcined cadmium sulfide precipitate. Thus,

a cadmium sulfide precipitate may be prepared in known manners without any selenium compound being present during the strike, and then the desired amount of selenium oxide may be stirred into the precipitate (preferably after the latter has been filtered and washed), together with added Water if necessary, until the selenium oxide has all been dissolved and has been uniformly disseminated through the precipitate. The whole mass may then be dried and calcined, Selenium oxides and/or other selenium compounds may also be mixed in a dry statewith dry cadmium sulfide greencake until a substantially uniform mixture has been secured, after which the dry mixture may be calcined.

In the course of our efiorts to use selenium oxides directly in preparing cadmium red pigments from cadmium yellow greencekes, we found that selenium oxides by themselves can provide too much oxygen for efficient 2,777,778 Patented Jan. 15, 1957 It will be noted that according to this equation the added sulfur is proportioned so as to form S02 with half of the oxygen introduced in the form of selenium dioxide, the remaining oxygen thereof ultimately forming $02 by combining with sulfur of the cadmium sulfide precipitate. Such proportions have been found to be very effective for all shades of red, although we have also found that in some instances the sulfur need only be proportioned to react with 3040% or less of the oxygen of the selenium dioxide, i. e., %-80% of all oxygen in excess of one atom of oxygen per atom of selenium. While the selenium is effectively combined when such smaller proportions of sulfur are used, the amount of soluble cadmium found in the calcined pigment generally tends to increase as the proportion of sulfur is reduced below the amount corresponding to about 40% of the oxygen content of the SeOz. Accordingly, to prevent such loss of cadmium, we prefer to use amounts of sulfur corresponding to at least 40% of the oxygen content of the SeOz, and We especially prefer to consume 50% or thereabout, i. e., substantially all of the oxygen in excess of one atom thereof per atom of selenium.

Other reagents may be employed similarly in place of sulfur; for example, SeS or SeSz. Elemental selenium may also be employed along with S602 in equal molar proportions, since such proportions provide only one atom of oxygen per atom of selenium present. Likewise combinations of elemental selenium and selenium trioxide may be used similarly,

If oxidic compounds of selenium other than selenium oxide(s) are used, such as selenium sulfite (SeSOa), or the selenyl halides (SeOClz, SeOBrz, SeOFa or SeOIz),

blue with excess oxygen can be reduced materially and in some instances eliminated completely, since added sulfur is only needed when the oxidic selenium compound of sulfur (if any) present in the compound. Thus no added sulfur is needed in the case of such compounds as SeSOz and the selenyl halides.

It should be understood that more sulfur or more sulfur-providing compound than is needed to combine with half of the oxygen (i. e., all of the oxygen in excess of one atom thereof per atom of selenium), of SeOz'may be used, but that such excess sulfur would lighten the shade of red over the shade secured in the absence of such excess.

The usual ranges of temperature employed in prior art calcinations may be employed when practicing our invention, but we have found that in general the optimum temperatures conducive to maximum tinting strength and to maximum utilization of selenium are of narrower ranges than in prior art practices. In other Words, where intervals of 50 C. might be tried in investigating calcining ranges of prior art practices, one should employt al in a sulfuric acid-nitric acid solution, containing 197 g. of cadmium per liter and of 5.9 and a specific gravity of 1.335 at 25 dium sulfide solution was prepared by dissolving flake impurities.

the final pH was 8.5. The precipitate was not readily obtainable in this way.

' water.

, be conducted in an essentially inert or mildly reducing atmosphere, as in the prior art, and that the hot pigment should be quenched in Water to minimize oxidation of the pigment and deterioration of its color.

EXAMPLE 1 A pure cadmium yellow greencake was prepared by a simultaneous strike of a cadmium sulfate solution with a solution of sodium sulfide.

The cadmium sulfate soludissolving electrolytic cadmium' metto give a solution having a pH C. The sotion was prepared by sodium sulfide in water and filtering to remove insoluble The resulting solution contained 34.5 g. of

sulfide ion per liter, had a specific gravity of 1.084 at 25 C. and a Baum density of 11.3. Thirteen hundred milliliters of the above sulfate solution were struck at room temperature with 2107 milliliters of the sulfide solution by simultaneously introducing the solutions into 511 milliliters of a sodium sulfate solution having a concentration of sodium sulfate of 100 grams per liter. The precipitation pH was maintained in the range 8.2-8.5, and was filtered and washed to remove the soluble salts, and was then dried.

' A dry mixture of yellow greencake as prepared above and selenium dioxide was ground in a mortar and calcined. Upon examination of the calcined product, is was apparent that a thorough dispersion of selenium dioxide There were dark specks in the pigment attributable to localized high concentrations of selenium dioxide. Ball milling or other means for securing a more intimate mixture overcome this difiiculty.

EXAMPLE 2 Selenium dioxide was dissolved in water to form selenious acid and the acid solution was added to dry yellow greencake of Example 1 to form a paste. The amount of selenium introduced in the form of selenious acid was such as to yield a red pigment containing approximately 17% selenium. The paste was dried and pulverized, and was then calcined at 600 C. The pigment was slightly brown and there was soluble cadmium salt in the quench water, indicating an excess of oxygen in the mixture which was calcined.

EXAMPLE 3 A portion of the yellow greencake of Example 1 was slurried in water and selenium dioxide was added in the proportions employed in Example 2. Flowers of sulfur were also added in an amount calculated to convert to sulfur dioxide half of the oxygen content of the added gave no soluble -minutes at 630 C.,

. gen.

C.; tint-200; ML30% est tinting strength was produced at 550 color 65% ML35% MR; mass tone 70% MR; the pigment was clean and bright.

EXAMPLE 4 A mixture was prepared to duplicate that of Example 3, except to include (NHQzl-IPOa. equivalent to 1% Color: 70% ML-30% MR; very clean and bright Mass Tone: 60% ML-40% MR 50 C.: it

Tint: 260 Color: 55% MR--% ML; clean and bright Mass tone: 80% MR--20 %ML It will be noted that the conditioner increased the tinting strength and deepened the shade. (NH4)2HPO4 equivalent to 1% HsPOi 0n the total solids is included in each of the following examples.

EXAMPLE 5 A mixture intended to produce alight red pigment was prepared by adding an aqueous solution of selenium dioxide to a portion of the yellow greencake of Example 1 in proportions of .247 grams of selenium per gram of cadmium sulfide. The resulting mixture was dried and pulverized and then was calcined. When calcined 35 minutes at 610 C. and quenched, the quench water contained appreciable amounts of cadmium salts. The resulting pigment had a tinting strength of approximately 250, and a clean color of LR50% ML. When calcined 30 less soluble cadmium was found in the quench water, and the pigment had a tinting strength of approximately 230 and a clean and bright colorof 30% LR--70% ML.

When a mixture was made up with the same proportions of selenium dioxide but with enough sulfur to combine with half of the oxygen of the selenium dioxide, calcination at 560 C. for 36 minutes gave no soluble cadmium, a tinting strength of 300, and a color of 45% LR-% ML. Calcination at 600 C. for 26 minutes cadmium, a tinting strength of 240, a clean color of LR--40% ML, and a clean mass tone of 40% LR-60% ML.

EXAMPLE 6 Two pastes intended to produce medium red pigment were prepared from portions of the yellow greencake of Example 1 with added aqueous selenium dioxide solution and elemental sulfur. The selenium dioxide was proportioned to yield .445 gram of selenium per gram of cadmium sulfide. One of the pastes, identified as Paste A, contained enough sulfur'to react with 37% of the oxygen of the selenium dioxide, while the other paste, paste B, contained enough sulfur to react with 50% of the oxy- After being dried and pulverized, the respective selenium dioxide. The slurry was thoroughly mixed, 60 crude pigments were each calcined at 525 C. and 550 evaporated to dryness, and pulverized. The pulverized C. The results are tabulated:

Paste Ten1p Time Soluble Cad. Tinting Color Mass Tone 0. Strength 525 25 None 280-300 Approx. MR-Brow'n Approx. MR-Brown. 550 21 Very Slight-.- APPIOLZSO--- 80% MR-20% ML 71;, gift-55%? (possibly s1g y town 525 17 None 285 55% MR--45%D 40% NIB-450% D. 550 21 .do 275; 60% MR40%D Clean and bright 20%MR- product was calcined the range 550600 EXAMPLE 7 Various pastes intended to give maroon pigments were made up from portions of the yellow greencake of Example 1 by adding aqueous selenium dioxide solutions and flowers of sulfur thereto, and then drying, pulverizing stantial part of the desired selenium content ofan inand calcining. The proportions employed are tabulated tended cadmium red pigment in the form of an oxidic along with the indicated data of each test: selenium compound. Preferably at least about half of V Cale. Time Solub Tintin C0101 Mass Tone Temp, 0.. Cad. Strengt Paste .444 g. SeOz/g. CdS; 16.8% 02188017811 with S 550 28 None Approx. 300. Darker than maroon Darker than maroon-brown.

612 Slight"- 300 Dgrker t)han maroon (possibly Darker than maroon (possibly brown).

rown 635 17 do Approx. 260-.- Maroon (possibly brown)-.-" Do.

Paste D; .444 g. .SeOz/g. OdS; 30% Oz reacted with S 585 30 None 265 45% M-55% D (clean) 60% M40% D; clean and bright. 600 23 Very Slight..- 250 55% lug-% D (cleaner than 60% M40% D; cleaner than standard. s an ar 1 Paste E; .444 g. SeOn/g. (MS; 35% O2 reacted with S 585 32 None 260 M50% 13---; I 80% M2o% D; very slightly brown.

Paste F; .433 g. Se02/g. (MS; 41% 02 reacted with S Very slight. Slight 3Z5 e D40% M; clean. 3 0

Deep-Brown 80% D 20% M; clean.

Paste G; .433 g. SeOz/g. CdS; 50% O2 reacted with S 320 55% MIR-45% D 290 560 35 N egligible 32 7 None MR30% D; clean Paste H; .467 g. SeOz/g. CdS; 50% Oz reacted with S 31 Very slightfifl 325 253,3 M% D; Very slightly M-20% D; equal to standard.

row-n. 270 30 N one 40% ill-60% D; equal to standard. 580 30 do 270 10% M-% D; clean 20% M80% D; clean.

EXAMPLE 8 l l the desired selenium content is so introduced, this being Extended cadmium red 7 40 the approx mate amount employed when a mixture of Elemental sulfur was added to a barium sulfide solution a crude pure cadmium sulfide pigment. When an oxidic in sufficient quantity to react with 50% of the oxygen compound such as SeSOs or SeOClz is used, all of the in the SeOz added subsequently to the washed precipidesired selenium can be introduced by the compound. tate. Likewise, when SeOz is used with or withontenongh .ele- The preparation was as follows: mental sulfur to react with up to 50% of the oxygen 555 'cc. of cadmium sulfate solution was placed in a content of the selenium dioxide, then all of the desired beaker provided with rapid agitation; The solution anaselenium can be derived from the SeOz. However, if a lyzed: compound such as SeS or SeSz is used to provide the cd=180 5o reducing component which reacts with up to 50% of the PH+=5 9 oxygen content of the SeOz, or if a solution of selemum Barium sulfide solution of the following composition selenium comes from an oxidic selenium compound, and

was prepared: the balance comes from the SeS, SeSz or selenyl chloride and dissolved selenium. Thus, in its broadest aspects, g gg g gfi 35 5 59 the invention departs from the prior art by contemplating Baum at C b 4 the introduction intoa pure or extended uncalcined cads cifi 1 mium sulfide precipitate of at least a substantial part, pe c gm lty and as much as all of the desired selenium in the form 855 cc. of the B218 solution was treated with 8.24 g. of an oxidic selenium compound.

elemental sulfur and agitated to effect complete solution. 60 In summary, it will be apparent that the present inven- The sulfur-treated barium sulfide solution was added' tion provides a unique and simple process for preparing to the cadmium sulfate solution, thereby precipitating cadmium red pigment, the process being eminently satiscadmium sulfide and finely divided sulfur. The precipifactory in producing satisfactory pigment qualities and in tate was washed and, in paste form, was treated with 57.1 utilizing the selenium efiiciently. The process makes it grams SeOz dissolved in a minimum of water. The paste 65 possible to eliminate the co-precipitation practices of the pone pigment of good tinting strength in the medium to sulfide solution, and to prepare any desired shade of pure deep range. or extended red pigment directly from a corresponding In the foregoing examples the abbreviations ML, MR pure or extended cadmium yellow greencake. Thus, in and D represent respectively medium-light, medium 7 the pigment plant, it becomes possible to precipitate only red and deep in relation to the various shades of red the pure or extended yellow greencake, and then to calsought. Also the numerical values of tinting strength cine those greencakes either directly to yellow pigment are in percent on standard base colors. (where such calcination is necessary), or, after incor- It will be understood that in its broadest concept this porating a desired amount of oxidic selenium compound invention contemplates the introduction of at least a sub- 75 with or without added sulfur or equivalent sulfur compound, calcining directly to red pigment. 'Hence only two precipitation procedures need be practiced, regardless of the ultimate color desired, and process control and personnel training problems are greatly reduced. Moreover,

the simplified procedures yield excellent pigment as the foregoing examples illustrate.

In the following claims, the term cadmium red pigment is employed to mean a cadmium pigment which in addition to cadmium sulfide contains at least enough selenium to impart to said pigment a color falling at least partially in the red range of the visible spectrum, md absent from said pigment when the said selenium content thereof is omitted.

Having now described the invention, what we claim is:

1. The process for which comprises: providing a finely-divided crude pigment mass composed essentially of (a) cadmium sulfide greencake, (b) selenium dioxide and (c) reducing agent selected from the group consisting of selenium, sulfur, binary selenium-sulfur compounds and mixtures thereof; said selenium dioxide being present in such quantity that the selenium thereof plus any selenium in said reducing agent together provide all of the selenium required to convert said greencake to a red pigment of desired shade, and said reducing agent being present in a total amount sufficient to form S02 with at least one-third of any oxygen of said selenium dioxide which is in excess of one atomic equivalent thereof for each atomic equivalent of the total selenium in said crude pigment mass; and calcining said crude pigment mass to effect its conversion to cadmium red pigment.

2. The process as claimed in claim 1 wherein the amount of selenium dioxide is such as to provide all of the required selenium, wherein sulfur is the reducing agent, and wherein the amount of sulfur is such as to form sulfur dioxide with between 60% and about 108% of the said excess oxygen content of said selenium dioxide.

3. The process as claimed in clah'n 2 wherein the amountof sulfur is such as to form sulfur dioxide with about 100% of the said excess oxygen content of the selenium dioxide.

4. The process as claimed in claim 1 wherein half of the required selenium is derived from selenium dioxide and wherein the remaining half is introduced as elemental selenium functioning as reducing agent.

SLThe process for preparing cadmium red pigment which comprises: providing a finely-divided crude cadmium pigment mass in which susbtantially all of the preparing cadmium red pigment ment obtained by calcining said mixture;

cadmium is-in the form of cadmiumsulfide; uniformly mixing with said crude mass at least one oxidic selenium compound in an amount sufiicient to'provide aratio of Se to CdS conducive to the shade of red desired in the pigand calcining said mixture to etfect its conversion into cadmium red pigment; said oxidic selenium compound being selected from the group consisting of the selenium oxides, the oxidic selenium acids, the selenyl halides and selenium sulfite.

6.The process for preparing cadmium red pigment which comprises: providing a finely-divided crude cadmium pigment mass composed essentially of cadmium yellow greencake, a substantial amount of selenium in the form of at least one oxidic selenium compound selected from the group consisting of the selenium oxides, the oxidic selenium acids, the selenyl halides and selenium sulfite, and at least a small amount of reducing agent selected from the group consisting of selenium, sulfur, binary selenium-sulfur compounds and mixtures thereof, said selenium in the form of oxidic selenium compound together with any selenium contained in said reducing agent being sufiicient to produce all the selenium needed to provide a desired shade of red in the pigment obtained by calcining said crude mass; and calcining said crude mass to effect its conversion into cadmium red pigment.

7. The process as claimed in claim 6 wherein the amount of reducing agent is suificient to combine with between about one-third and all of the excess oxygen contained in said oxidic selenium compound overthe amount represented by one atomic equivalent thereof for each atomic equivalent of selenium present in said crude pigment mass. 7

8. The process as claimed in claim 7 wherein the re ducing agent consists of sulfur coprecipitated with said cadmium yellow greencake.-

References Cited in the file of this patent UNITED STATES PATENTS Marcot July 18, 1950 Allan et al June 23, 1953 OTHER REFERENCES on Inor- Green 8: 

1. THE PROCESS FOR PREPARING CADMIUM RED PIGMENT WHICH COMPRISES: PROVIDING A FINELY-DIVIDED CRUDE PIGMENT MASS COMPOSES ESSENTIALLY OF (A) CADMIUM SULFIDE GREENCAKE, (B) SELENIUM DIOXIDE AND (C) REDUCING AGENT ELECTED FROM THE GROUP CONSISTING OF SELENIUM, SULFUR, BINARY SELENIUM-SULFUR COMPOUNDS AND MIXTURES THEREOF; SAID SELENIUM DIOXIDE BEING PRESENT IN SUCH QUANTITY THAT THE SELENIUM THEREOF PLUS ANY SELENIUM IS SAID REDUCING AGENT TOGETHER PROVIDE OF THE SELENIUM REQUIRED TO CONVERT SAID GREENCAKE TO A RED PIGMENT OF DESIRED SHADE, AND SAID REDUCING AGENT BEING PRESENT IN A TOTAL AMOUNT SUFFICIENT TO FORM SO2 WITH AT LEAST ONE-THIRED OF ANY OXYGEN OF SAID SELENIUM DIOXIDE WHICH IS IN EXCESS OF ONE ATOMIC EQUIVALENT THEREOF FOR EACH ATOMIC EQUIVALENT OF THE TOTAL SELENIUM IN SAID CRUDE PIGMENT MASS; AND CALCINING SAID CRUDE PIGMENT MASS TO EFFECT ITS CONVERSION TO CADMIUM RED PIGMENT. 